Wireless charging modes have been developed for the charging of portable devices in which the portable device includes a coil for receiving or transmitting power wirelessly to a coil in an external device. Due to the less-than-ideal coupling between such coils, the portable device requires a relatively high-power supply voltage such as 15 V to energize its coil when transmitting power to another device. But the battery voltage of a mobile device is typically substantially lower voltage such as 4 V. A mobile device thus needs a power converter to boost the battery voltage to the elevated power supply voltage necessary for wireless power transmission.
One choice for such power conversion is a two-stage power converter in which a boost converter boosts the battery voltage to an intermediate voltage that is then doubled by a flying-capacitor-based charge pump. The resulting two-stage power converter has advantageous efficiency and regulation. But during certain modes of operation such as startup and shutdown, the charge pump is not switched but instead has its input shorted to its output such that the charge pump is bypassed. This mode of operation is denoted herein as a bypass mode of operation. During the bypass mode of operation, it is only the boost converter that is driving the output voltage for the two-stage converter. There will thus be a first transition from the bypass mode to a normal mode of operation in which charge pump is active. Similarly, there is a second transition from the normal mode of operation to the bypass mode of operation. In both transitions, the output voltage may have significant overshoot and undershoot of the desired regulated value.
Accordingly, there is a need in the art for charge-pump-based two-stage converters having improved bypass mode transitions.